There is currently a need to fabricate integrated circuits with extremely small (less than one micron) dimensions. In cases where the wafer topography is uneven or nonplanar, optical imaging problems limit the dimensions possible. Additional problems are caused by the nature of wet etch or development methods. For example, sloped sidewalls of the resist image cause lower image contrast and critical dimension (CD) control in image transfer to an underlying layer.
To meet this problem, trilayer microlithographic processes have been developed. One example is disclosed by P. S. Burggraaf ("Multilayer-Resist Lithography", Semiconductor International (June 1983)) who describes a method for patterning an integrated circuit by first forming a planarization layer of organic material with a depth sufficient to allow a substantially planar outer surface. A second, masking resist layer is spin-deposited on the first layer followed by the deposition of a third imaging layer. The imaging layer is selectively patterned and the pattern is transferred to the masking layer which is then developed. This is followed by an oxygen-based reactive ion etch (RIE) etch to transfer the pattern to the surface of the workpiece.
Several problems are present with the trilayer approach. It is often difficult to remove the second masking layer due to its hardened nature. Also, the creation of undesired particulate matter lowers throughput yields. In addition, although the imaging layer will reduce the depth of field by as much as a factor of two, the technique does not utilize the limits of the optical imaging tool.
Another approach, described by B. Roland et al. is the DESIRE.TM. process. Here a thick, planarizing layer of resist is deposited on the wafer. Silicon is then selectively incorporated from the gas phase into the photo-exposed areas after coating and photo-exposure of the resist. In an oxygen plasma, the incorporated silicon will form an oxide mask that stops etching in the photo-exposed areas.
One problem occurs because a production-worthy deep ultraviolet (UV) resist has yet to be developed. A significant problem with current experimental resists is related to the exposure sensitivity. The light doses, in units of millijoules per centimeter, required to create an image are currently unacceptably high. Low sensitivity results in low throughput and poor image contrast. Moreover, irrespective of the light dosage, resultantly developed image profiles are not acceptable. Also, with this method, the process latitude is not sufficient to meet fabrication process latitude requirements.